Use nproc and sysctl -n hw.cpu for default initializing CORES

This avoids having to pass --build-arg CORES=n when building the
docker images.

Dockerfile.cross

@@ -22,8 +22,6 @@ ENV CROSS_TOOLCHAIN_PREFIX=/opt/llvm-mingw-$CROSS_ARCH
 ENV EXEEXT=.exe
 ENV HOST=$CROSS_ARCH-w64-mingw32
 
-ARG CORES=4
-
 COPY build-llvm.sh .
 RUN ./build-llvm.sh $CROSS_TOOLCHAIN_PREFIX
 COPY strip-llvm.sh .
@@ -47,7 +45,7 @@ RUN wget -q https://ftp.gnu.org/gnu/binutils/binutils-2.31.1.tar.xz && \
     tar -Jxf binutils-2.31.1.tar.xz && \
     cd binutils-2.31.1 && \
     ./configure --host=$HOST --target=x86_64-w64-mingw32 --prefix=/opt/binutils LDFLAGS="-Wl,-s" && \
-    make -j$CORES && \
+    make -j$(nproc) && \
     make install && \
     cp /opt/binutils/x86_64-w64-mingw32/bin/objcopy.exe $CROSS_TOOLCHAIN_PREFIX/bin/binutils-objcopy.exe && \
     cp /opt/binutils/x86_64-w64-mingw32/bin/strip.exe $CROSS_TOOLCHAIN_PREFIX/bin/binutils-strip.exe